FI85133B - FOERFARANDE FOER FRAMSTAELLNING AV AKTIVKOL FRAON CELLULOSAHALTIGT MATERIAL. - Google Patents

Abstract

Method for manufacturing activated carbon from cellulosic material which method comprises treatment of the material with a Lewis acid and a subsequent pyrolysis and activation in a suitable atmosphere and using a suitable temperature-time program. The material is brought into contact with lithium ions prior to pyrolysis whereby a larger specific surface area and a better strength is obtained.

Description

1 85133

The invention relates to a process for the preparation of activated carbon from a cellulosic material, which comprises treating the material with a Lewis acid followed by pyrolysis and activation in a suitable atmosphere and using a suitable temperature-time program.

This method is presented e.g. GB patent 1301101 and DE patent 2915640. It is known to form activated carbon from a cellulosic fibrous material by subjecting the fibers to pyrolysis and activation in a suitable environment by raising the temperature by a suitable program to between about 500-1000 ° C. In this heat treatment, on the other hand, a certain degree of weight loss occurs due to pyrolysis and at the same time the microporous nature characteristic of activated carbon is created.

In order to reduce weight losses, the methods disclosed in the above patents have been developed by first treating a fibrous starting material of activated carbon, such as a fabric, with a solution containing Lewis acid, such as a solution containing aluminum chloride, zinc chloride or calcium chloride, or a mixture thereof.

Indeed, the above methods have been able to significantly improve weight loss at a later stage, resulting in better strength of activated carbon products made from fibrous cellulosic material, such as activated carbon fabrics. In this case, however, some other optimal properties important for activated carbon, such as porosity, have not been achieved.

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The object of the invention is to provide a method by which the properties of activated carbon made of a cellulosic material, in particular porosity, but also other properties can be improved. To achieve this purpose, the process according to the invention is mainly characterized in that, prior to pyrolysis, the material acting as a raw material for activated carbon is brought into contact with lithium ions.

The invention is based on the surprising finding that lithium works excellently in the cellulosic structure of the raw material together with Lewis acids. Lithium has been found to improve yield, enhance the action of Lewis acids, and in particular increase the specific surface area without compromising the strength of the product.

The cellulosic material to be pretreated is preferably contacted with lithium ions by impregnating the material with a solution containing lithium ions. This impregnation is preferably carried out in the same step as the treatment with Lewis acid, i.e. the treatment solution contains, in addition to the Lewis acids, a salt comprising lithium ions, such as lithium chloride.

Many types of cellulosic fiber can be used as the cellulosic starting material, but in particular regenerated cellulosic fiber, such as viscose fiber, has proven to be a practical raw material due to its good initial strength and suitable initial porosity. 30 High-tenacity viscose fiber, known per se, can then be used as a raw material. For example, prior to treatment with Lewis acids and lithium chloride, the viscose fibers can be formed into a fabric, whereby the final activated carbon product is also in the form of an activated carbon fabric.

In the following, the process steps belonging to the method according to the invention and the phenomena occurring in them, which affect the properties of the final product, are described in more detail.

The starting material used is a material consisting of cellulose fibers, which can be, for example, in the form of a fabric. The preferred starting material is, for example, viscose fibers, which have an inherently good strength and a certain degree of porosity. A suitable material is, for example, high-10 tenacity viscose fibers of 1.0-1.3 dtex, manufactured by the applicant, with a tensile strength of about 2.9-3.2 cN / dtex. The initial porosity of the fibers measured with a mercury porosimeter is in the range of 100 to 500 μΐ / g.

A fabric formed from the fibers is woven by a standard method into a fabric which acts as a starting material for an activated carbon fabric. In the pretreatment, the fabric is impregnated with a solution containing Lewis acid. The solution may contain, for example, aluminum chloride, calcium chloride, ammonium chloride or zinc chloride or a mixture thereof. The total concentration of these substances in the impregnation solution is generally 2 to 10% by weight. For example, a mixture of Lewis acids with approximately equal percentages of salts selected from zinc chloride, calcium chloride, aluminum chloride and ammonium chloride may be used. Lewis acids have been found to have a beneficial effect on yield in pyrolysis, i.e. the carbonization step, and on the strength of the final product. Lewis acids have been found to modify the thermal decomposition mechanism of the cellulosic fiber used as a starting material, so that the main reaction in carbonization is dehydration, i.e. the cleavage of water, with the yield approaching under theoretically favorable conditions. In addition, it has been speculated that the improved strength is due to the presence of metal ions in the structure of the product.

According to the invention, the starting material is treated with lithium ions before pyrolysis so that the ions can act on the structure of the substance. Lithium has been found to have a surprisingly good effect on both yield and specific surface area. This effect is thought to be due in part to the fact that lithium ions, when penetrating between cellulose chains, break down intermolecular hydrogen bonds, allowing Lewis acids to act better on cellulose molecules. Lithium also prevents the formation of hydrogen bonds during the drying step. When carbon dioxide is used as the activating gas in the activation step, lithium further improves microporosity by increasing the efficiency of the carbon dioxide. This is believed to be related to the carbon dioxide properties of lithium. Lithium is used e.g. in the soft drink industry in carbonated beverages to reduce carbon dioxide evaporation.

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Still other properties of lithium make it quite suitable for the pretreatment of another material in the production of activated carbon. Compared to alkaline earth metals and other alkali metals, it has both the smallest atomic radius and the smallest covalent radius.

For comparison, the atomic radii (left values) and covalent radii (right values) of alkali metals as angstroms are shown below.

25 Li 2.05 1.23

Na 2.23 1.54 K 2.27 2.03

Rb 2.98 2.16

Thu 3.34 2.35 30

Due to their small size, lithium ions can penetrate well into the structure to be treated in connection with the cellulose chains.

In addition, due to its physical properties, lithium chloride retains well in the structure of the starting material without evaporation from it at the high temperatures of the pyrolysis and activation steps.

Li 5 85133 The treatment with lithium ions is carried out in the same step as the treatment with Lewis acids, wherein the aqueous treatment solution contains a suitable 5 Lewis acid composition and a suitable concentration of a water-soluble salt of lithium, preferably lithium chloride. The treatment is preferably carried out by immersing the starting material in the treatment solution, but alternative ways of impregnating the starting material with the treatment solution, such as spraying, are also possible. The contact residues and treatment temperatures can be selected so as to obtain the desired concentration of active ingredients (Lewis acids and lithium ions) in the starting material.

After the treatment, the starting material is dried mechanically by pressing and then by heat at a temperature of 80 to 120 ° C. After drying, the starting material formed from the fiber-salt mixture is mechanically softened so that its final flexibility is not affected.

The actual preparation into the final activated carbon product is performed with a specific temperature-time program in a suitable atmosphere. The starting material is placed in the furnace 25 so that it is allowed to shrink freely during the heat treatment. In the initial stage, pyrolysis, i.e. carbonization, is performed, in which case the actual weight losses occur and the carbon body of the final product is formed. The carbonization is carried out in a suitable protective atmosphere, e.g. carbon dioxide or nitrogen at a temperature of about 300 to 450 ° C.

The activation is carried out immediately thereafter in a carbon dioxide atmosphere at a temperature of about 500 to 1000 ° C, whereby a final activated carbon product with the desired porosity is formed. Em. In addition, an important factor in the processes is the rate of temperature rise and residence times at certain temperatures critical for product formation.

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The following describes a series of experiments performed by the method according to the invention.

A fabric (MS fabric) having a basis weight of 186 g / m 2 was woven from a 15 tex viscose yarn (yarn 2804) formed from the above-mentioned high-tenacity viscose fibers. The fabric pieces were pretreated according to the invention with various solutions containing Lewis acid and lithium chloride, in addition to which a comparative treatment was performed with solutions without lithium. The compositions and pH of the aqueous solutions were as follows:

Solution 1: 1.5% AlCl 3, 3.0% CaCl 2, 3.0% NH 4 Cl, pH 3.4 Solution 2: Lewis acids as in solution 1, plus 3.0% LiCl, pH 3.2

Solution 3: 3.0% CaCl2, 3.0% NH4Cl, 3.0% ZnCl2, pH 5.2 Solution 4: Lewis acids as in solution 3, plus 3.0% LiCl, pH 4.5

In addition, two alternative pretreatment programs were used: A) The solution was allowed to impregnate a piece of cloth while raising the temperature of the solution from 40 ° C to 80 ° C at a rate of 10 ° C / min, after which the piece was kept in 80 ° C solution for 2 min.

B) The solution was allowed to impregnate a piece of cloth for 10 min at room temperature.

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In both cases, the excess solution was removed mechanically after treatment by first passing the piece successively five times between the press rolls using a suitable line pressure, most preferably 35 18 kp / cm, after which the piece of fabric was dried at 110-120 ° C for 2-5 min. After drying, the piece of fabric was further mechanically softened by passing it three times through press rolls, most preferably using the above-mentioned line pressure.

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After drying and softening, the piece of fabric was placed on a thermal balance in an oven so that it could shrink freely during pyrolysis and activation. In the pyrolysis step, the oven temperature was raised at a rate of 5 ° C / min from 20 ° C to 360 ° C, where the burn was held for 15 min. The temperature was then raised at a rate of 10 ° C / min to 900 ° C, and the sample was kept at this temperature until the mass loss did not increase by more than 20% during the activation step. The furnace's protective air-10 perimeter and activation atmosphere was carbon dioxide.

The results of the test series are shown in the following table.

Table

Properties of activated carbon fabrics made with different treatment solutions. Solutions 2 and 4 contain lithium.

Solution / pretreatment yield (%) BET (m2 / g) DPW DPn PD strength 0 - - 2169 853 2.57 1 / A 19 520-650 830 369 2.25 brittle 1 / B 21 580-670 899 384 2 , 34 moderate 2 / A 18 730 799 364 2.20 good 2 / B 20 790-810 1035 458 2.26 good 3 / A 18 520 2285 855 2.67 fragile 3 / B 18 550 2373 910 2.61 moderate 4 / A 20 800 2135 907 2.35 good 4 / B 22 820 2253 935 2.41 good 0 = starting material BET = specific surface area DPW = mass average degree of polymerization DPn = number average degree of polymerization PD = DPw / DPn = polydispersity

Due to the small size of the sample, the strength properties have been determined by manual testing.

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As can be seen from the experimental results, lithium ions have an increased specific surface area (BET) at different Lewis acid compositions and pretreatment temperatures. The yield in pyrolysis and activation can be kept the same, or it can even be slightly increased (differences between the products obtained with solutions 3 and 4), which is related to the strength properties of the final product.

The invention is not limited above only to the alternatives described in the description and series of experiments, but may be modified within the scope of the inventive idea set forth in the claims. The starting materials are all cellulose-based materials used as starting materials for activated carbon, in which the phenomena described above, which can be favorably influenced by lithium, take place during the manufacturing steps. When regenerated fibers are used as starting materials, lithium ions can be incorporated into the cellulosic structure by adding a salt containing them, such as lithium chloride, to the solution even before the regeneration of the cellulose. However, by treating the regenerated fibers with a lithium-containing solution, it is possible to carry out the treatment more economically.

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Due to their good strength and absorption efficiency, the activated carbon products produced by the process according to the invention have many practical applications related to the separation of substances from gases and liquids, of which respirators and various filters can be mentioned in particular.

Claims (6)

A process for preparing activated carbon from cellulosic material, comprising treating the material with a Lewis acid and subsequent pyrolysis and activation in a suitable atmosphere and using an appropriate temperature-time program, characterized in that the pyrolysis material is contacted with lithium ions. Process according to claim 1, characterized in that the material is contacted with lithium ions by impregnating the material with a solution containing lithium ions. 15 Process according to claim 2, characterized in that the Lewis acid-containing solution contains lithium ions comprising salts, such as lithium chloride, the treatment with the lithium ions being carried out simultaneously with the Lewis acid treatment with the solution. Method according to claim 2 or 3, characterized in that the amount of lithium in the solution is above 1.0% by weight, preferably 2.0-5.0% by weight, calculated as lithium chloride. Process according to any of the preceding claims, characterized in that regenerated cellulose fiber such as viscose fiber serves as the cellulosic material. Process according to any of the preceding claims, characterized in that the material is in the form of fabric. li